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Alasdair Beal [09/02/10]

Report 166 (Newsletter No 17 assets/uploaded/fck/File/CROSS Newsletter 17[1].pdf) concerning the collapse of a freestanding boundary wall misses the main problem with these walls, which is that they are not covered by Building Regulations (unless they form part of a building), so their construction is unregulated. Therefore local authority building control officers have no authority to do anything about the design or condition of such a wall unless someone reports it as a dangerous structure (by which time it is usually too late).

Brian Bell [04/01/10]

To supplement Allan Mann’s points (see previous Feedback item), I would add that active signalling may be considered in conjunction with passive structural resistance where catastrophic loss of a bridge span caused by a low probability event may be accepted in some cases as being too expensive to avoid completely. While it smacks structurally of giving up the ghost or throwing in the towel, actually it is a pragmatic solution to a very difficult matter, particularly in low income economies where a remote bridge may be crucial to a locality and be heavily trafficked in the dark or in very poor visibility (e.g. monsoon conditions, which can wipe out a bridge in a very short time.) The principle need not only apply to bridges, of course. Below is a reference to a system which is a high-tech variant of the weighted fusible string low-tech solution: either can be deployed in most parts of the world.

If de la Concorde Overpass (30 Sept 2006: http://en.wikipedia.org/wiki/De_la_Concorde_overpass_collapse ) or I-35W (1 Aug 2007: http://en.wikipedia.org/wiki/File:35wBridgecollapse.gif ) had failed during the hours of darkness, the loss of life could have been substantially greater. 

James Justin Mercier, Eligio Alvarez, Juan Marfil, Mark J. Bloschock, and Ronald D. Medlock,

The Texas Department of Transportation (DOT) installed a collapse detection system on the Queen Isabella Memorial Bridge that will detect a span collapse and warn motorists to stop. The system consists of a fiberoptic cable that carries a current under the bridge deck for the 2½-mi length of the bridge. A span collapse will break the current, initiating flashing red lights to tell motorists on the bridge to stop, closing gates at each end of the bridge to keep additional cars off, and sending alarms to Texas DOT and local law enforcement to notify them of the event. 

(6th International Bridge Engineering Conference, July 2005, Boston , Mass. )

allan.mann [15/12/09]

All engineers will have noted the collapse of the bridges in Cumbria recently. Whilst there has been some discussion on the causes (possibly scour) there are wider safety lessons to be learned

1. The death of the policeman was precipitated by a sudden collapse. It is a fundamental target of structural engineering that any failure will be foreshadowed by signs of distress to give warning. The question therefore is how fast was this collapse, would there have been signs of distress (from this failure cause) and if not, does that suggest the margins of safety against (whatever caused the failure) need to be increased?

2. A second attribute of safety is that of assessing sensitivity. We all know there have to be best assumptions about loading conditions etc. And we all have heard that this was an 'exceptional event’. But in industries where statistically possible events (albeit rare) can occur yet the consequences are severe, the aim is still to preserve a safety margin (maybe > 1.0) against extreme events. The target being to survive short of catastrophic collapse. What is not acceptable is to have a rapid change of state consequent on a marginal exceedence of design conditions. To assess this, sensitivity studies are called for. Has anyone looked to see if these failures were explainable by undue sensitivity to the presumed accuracy of the design events?

3. In safety related plant it is always an objective to have the design ' inspectable'. Numerous failures have occurred with bad consequences simply because it was impossible to verify plant/ structure condition. Was the source of failure (say scour) such that the condition was impossible to verify? There is absolutely no point in fretting whether the bridge superstructure state is Ok to stress levels (where the consequence might be cracking) and spending scarce resources verifying this 'accurately'; if a catastrophic failure can be precipitated at foundation level but can't be verified in the field.

4. All failures are regrettable. But as is well known whenever there is a near miss the wider question should be what is the risk to the remaining structures? After serious bridge failures in US and China we know surveys predicted many thousands of bridges were at risk. It is commendable such surveys are being conducted in Cumbria to assess bridge state there. But surely these flooding events can happen in any county. What is the vulnerability of the UK bridge stock elsewhere? How much will an assessment exercise cost (however crude) against the full costs of a single bridge replacement? For all these reasons, a full and open enquiry into the bridge failure causes is merited.

Derek Bishop [27/10/09]

It is worth noting that (see Newsletter No 16 Report 158 - Substitution of Cold Rolled Hollow Sections) an alteration to the design (including specification) by 'another' person should be considered under Regulation 11 (ss 3 & 4) of the Construction (Design and Management) Regulations 2007 whereby the Buyer takes on the duties of the 'Designer' and should consider the risks to health and safety (in the construction process and use (of the structure)) that may arise from the alteration to the original specification.

Investigation, should a failure occur, could identify the reason for the failure and at what stage the change in the original specification took place and who was responsible for making that change.

Robert Jones [29/09/09]

May I welcome this development of the CROSS site and hope that it makes it easier for more colleagues to report situations where the safety of persons could be compromised. There is no statutory record of dangerous structures and “near misses”, CROSS is the only national register in existence as Dangerous Structures legislation is discharged locally with no other national record. Too often small issues that can lead to disaster are ignored, I had to rebuke a builder today for building a blockwork core off a first floor slab, before the supporting ground lift had been constructed under. An email to his office has had to confirm my concerns, and request support be provided, as I could be liable under section 3(2) of the HSW Act.

I feel privileged to be an Engineer and feel I have a duty to the public at large and my profession, so I seek to discharge that duty of care diligently, and would encourage colleagues to do likewise and support CROSS and the Institution in making buildings safer places. This might seem a little bit altruistic, but I regularly come across Engineers who would never dream of undertaking any action for fear of becoming involved.

Eur Ing Robert Jones DM CEng FIStructE FCIOB FBEng MCMI Chartered Structural and Corporate Building Engineer